A) Field of the Invention
The present invention relates to ZnO crystal and its growth method. The present invention relates also to a manufacture method for a light emitting device using ZnO.
B) Description of the Related Art
Zinc oxide (ZnO) is semiconductor of a direct transition type having a band gap of 3.37 eV at a room temperature, and has a large exciton binding energy of 60 meV (e.g., a binding energy of zinc selenide (ZnSe) is 18 meV and a binding energy of gallium nitride (GaN) is 24 meV). ZnO is expected as material of a high efficiency light emitting device. For example, in manufacturing a light emitting diode, it is necessary to obtain p- or n-type ZnO by doping impurities. For example, nitrogen (N) is doped as p-type impurities and gallium (Ga) is doped as n-type impurities.
ZnO crystal has a wurtzite structure having a +c plane (a plane exposing Zn surface, i.e. Zn polarity plane) and a −c plane (a plane exposing O surface, i.e. O polarity plane) relative to a c-axis direction. For example, technologies of growing ZnO crystal exposing the Zn polarity plane on a sapphire substrate (growing ZnO crystal with the Zn polarity plane) is disclosed in JP-A-2002-326895, the entire contents of which are incorporated herein by reference. JP-A-2002-326895 discloses that ZnO crystal doped with N during growth with the Zn polarity plane has a higher photoluminescence (PL) than ZnO crystal doped with N during growth with the O polarity plane.
It is therefore considered that ZnO crystal grown with the Zn polarity plane is more suitable for manufacturing a light emitting device than ZnO crystal grown with the O polarity plane.
Growth of ZnO crystal includes growth with a flat surface (two-dimensional growth) and growth without a flat surface (three-dimensional growth). For example, in manufacturing a light emitting device, it is preferable to two-dimensionally grow ZnO crystal.
Further, in order to grow p-type ZnO crystal suitable for light emitting devices, it is desired to use a crystal growth method capable of doping p-type impurities at a desired concentration. It is also desired to use a crystal growth method capable of having a desired n-type carrier concentration (electron concentration) in order to make n-type ZnO crystal suitable for light emitting devices.